1. Field of the Invention
The present invention relates to an image forming apparatus and an image adjusting method, based on an electrophotographic system. In particular, the invention relates to an image forming apparatus and an image adjusting method, capable of preventing degradation of image quality due to deterioration of toner properties by forcibly discharging the toner from a developing unit.
2. Description of Related Art
In an image forming apparatus using an electrophotographic system, there has been developed such technology that forms high quality images superior in image characteristics such as resolution, reproducibility of precise images, and makes it possible to form high quality images close to those printed by plate-using printing. As the print quality has become higher, a problem arises in degrading image quality due to deterioration of toner properties, and therefore measures for this problem have been studied.
It has been clarified that the toner deterioration occurs mainly when the toner remains in a developing unit for a long time, and the toner is stirred and carried in the developing unit to receive stress for a long time.
As a solution for this toner deterioration, such a technique has been developed that the toner in a developing unit is forcibly discharged and a new toner is supplied to exchange the toner (refer to patent Document 1: JP 2004-125829A). In this patent document, the number of rotation of a developing roller and the number of image dots are counted and stored, and if the number of image dots at a given number of rotation is less than a predetermined threshold value, then the toner is discharged from the developing unit.
According to the technique disclosed in patent Document 1, it has been found that degradation of image quality is somewhat prevented, but the image quality is not improved enough. Particularly, it has been found to be difficult to prevent degradation of image quality by prior art including the technique disclosed in patent Document 1, when images are formed at near life-time of developer, or under an environment of high temperature and high humidity. That is, according to the technique of Patent Document 1, it is difficult to keep track of the state of developer with high accuracy because of a control with prospects using the number of image dots.